J.H. Cloete

Tuning stubs for microstrip-patch antennas

A practical method for the simultaneous tuning, of both the resonant frequency and reflection coefficient, of a coaxially fed rectangular-microstrip patch is described. The use of two tuning stubs allows independent adjustment of the effective patch length and the effective position of the feed point. The effective length determines the resonant frequency of the patch, while the effective position of the feed point determines the input impedance. It is demonstrated that the method allows the adjustment of the feedpoint reflection coefficient of a patch to less than -60 dB, at a frequency which is within 0.005% of specification.<<ETX>>

Real-Time Sag Monitoring System for High-Voltage Overhead Transmission Lines Based on Power-Line Carrier Signal Behavior

A new method of measuring the change in the average height above ground of horizontal high-voltage overhead transmission-lines (OHTLs) phase conductors is introduced. The new technique, called power-line carrier sag (PLC-SAG) for short, determines average overhead conductor height variations in real time by correlating sag with measured variations in the amplitude of signals propagating between power-line carrier (PLC) stations. The multifrequency PLC-SAG monitoring signals are injected onto the operational PLC system in the 50- to 500-kHz band, but without interfering with the operational integrity of the PLC teleprotection system. The feasibility of the method was examined using the theory of natural modes for multiconductor systems, and tested by extensive field experiments on two horizontal 400-kV lines operated in South Africa by Eskom. It is concluded that the average height of a horizontal OHTL can be tracked accurately for continuous periods by the PLC-SAG technique.

Scattering and absorption by thin metal wires in rectangular waveguide-FDTD simulation and physical experiments

The high-frequency internal impedance model of a round ohmic conductor is incorporated into the subcell thin-wire formulation of the finite-difference time-domain method to model the microwave properties of metal wires. For magnetic metals, such as steel, an effective conductivity is introduced to account for the increase in ohmic loss due to the high-frequency permeability. Physical experiments with half-wave resonant copper- and steel-wire inclusions, supported by a dielectric slab in a standard S-band rectangular waveguide, support the formulation.

The Effect of Conduction on VHF Radar Images Shot in Water-Filled Boreholes

A downhole digital memory-logging pulsed borehole radar transceiver operating in the 10-125-MHz band was run repeatedly down two 1.25-km-deep, uncased, water-filled 60-mm boreholes in the Northern Limb of the Bushveld Complex, South Africa. Suspended on an insulating cord, it mapped a steep fault in the Main Zone from a range of 75 m down through its intersection with the borehole. Lowered on a wire rope, the transceiver launched guided ~75 m/mus first-order transverse magnetic pulses which shuttled axially between the radar and the bedding planes. Decoupled from the wire by 2 m of insulating cord, it yielded a profile in which radar reflections and guided bedding plane echoes superimposed. As the radar descended through the mineralized, stratified Platreef, traces were found to be imprinted with voltage level shifts that showed, with Laterolog-comparable resolution, the conductivity profile of the Platreef.

The breakdown electric field between two conducting spheres by the method of images

The determination of the breakdown electric field of air between two identical conducting spheres is a good example of the use of the method of images to solve a practical problem in engineering electromagnetics. The sparkover electric field is computed from published or measured data for the breakdown voltage by using the theoretical solution for the image charges. Thus students are exposed to a sophisticated method of images solution involving two sets of infinitely many charges. Additional educational benefits include an enhanced appreciation for the properties of air as an insulator, for the limits of the electroquasistatic approximation, for the numerical convergence properties of infinite series, and for the history of science. Instead of using published tables, an exposure to high-voltage engineering to obtain measured breakthrough data is an option if a suitable laboratory is available.

The role of chirality and resonance in synthetic microwave absorbers.

Summary Microwave absorption by a lossy dielectric material containing thin metal wires is considered. The wires are bent to create either chiral, non-chiral or racemic unit cells. No physical mechanism is found to support patents which were granted between 1990 and 1993, and related claims in the engineering literature, that chirality is the key to improved microwave absorbers. Instead, in synthetic composites which employ thin metal wires in a lossy dielectric host, half-wave resonance of the inclusions – not their geometric shape – is identified as the mechanism responsible for enhanced absorption. It is also found that – even in an essentially lossless host – resonant steel wires, whether chiral or not, can strongly absorb electromagnetic waves.

Advances in the design of Jaumann absorbers

The authors consider electric screens with the material spacers assumed homogeneous, nonmagnetic, lossless, commensurate, and isotropic. Normal incidence is assumed. Such an N-layered structure may be modeled as a transmission line network. The unknowns are the N spacer characteristic impedances and the N sheet surface resistivities. All are normalized by the free-space impedance. The maximally flat solution of Fante and McCormack (1988) is extended to any number of layers, with proven solvability up to 20 layers. An equiripple solution, which, for a given number of layers and specified maximum reflection coefficient, exhibits a wider bandwidth than the corresponding maximally flat solution, is described. The sheet resistivity spread is also much narrower, thus increasing realizability.<<ETX>>

S-parameter measurements yielding the characteristic matrices of multiconductor transmission lines

A frequency-domain method is presented, which yields accurate characteristic matrices of uniform multiconductor transmission lines (MTLs). It uses simple two-port network analyzer S-parameter measurements of a set of open-circuit and short-circuited MTL configurations. The method eliminates the need for voltage and current probes, which introduce errors. Transversely inhomogeneous MTLs can be accurately characterized in their quasi-TEM propagation regime. The influence of the skin effect on the inductance matrix is taken into account. The technique was used to determine the inductance and capacitance matrices of a low-loss three-conductor ribbon cable above a ground plane. Comparisons with numerically and analytically obtained data are given. Measurements are found to be repeatable for lines of length /spl Lscr/</spl lambda//4. The /spl lambda//4 requirement is not found to be a restriction in the megahertz regime and only plays a role as line-end effects become significant at gigahertz frequencies. The obtained accuracy is significantly better than previously reported results.

Bandwidth improvement of circularly polarised arrays using sequential rotation

The authors report on the sequential rotation technique to minimize the severe bandwidth limitations associated with circularly polarized patch antennas. The technique is applicable to all circularly polarized antenna rays. The theory and experimental results are discussed as applied to a patch array. It is confirmed that remarkable antenna bandwidth improvements may be obtained through the use of sequential rotation.<<ETX>>

A guarded cylindrical capacitor for the non-destructive measurement of hard rock core samples

The novel design of a flexible guarded capacitor which conforms to a core samples cylindrical surface is presented. The proposed device allows non-destructive measurements of hard rock samples produced by diamond drilling, and has numerous advantages over previous methods. No material is lost to sample preparation and a detailed characterization of the entire core, including inclusions and transitions between rock types, may be performed. A detailed methodology for the rapid and cost-effective construction of a robust capacitor is given. Guidelines for its operation to achieve repeatable and accurate measurements of the complex dielectric constant of samples of varying homogeneity in the 1–25 MHz frequency range are presented.